Swing I - Absolute Java - page 943

17.1 Event-Driven Programming

My duty is to obey orders.

THOMAS JONATHAN (STONEWALL) JACKSON

event-driven
programming

Event-driven programming is a programming style that uses a signal-and-response
approach to programming. Signals to objects are things called events, a concept we
explain in this section.

Events and Listeners
Swing programs use events and event handlers. An event is an object that acts as a
signal to another object known as a listener . The sending of the event is called firing
the event . The object that fires the event is often a GUI component, such as a button.
The button fires the event in response to being clicked. The listener object performs
some action in response to the event. For example, the listener might place a message
on the screen in response to a particular button being clicked. A given component may
have any number of listeners, from zero to several listeners. Each listener might respond
to a different kind of event, or multiple listeners might respond to the same events.
If you have read Chapter 9 on exception handling, then you have already seen one
specialized example of event-driven programming. 2 An exception object is an event.
The throwing of an exception is an example of firing an event (in this case, firing the
exception event). The listener is the catch block that catches the event.
In Swing GUIs, an event often represents some action such as clicking a mouse,
dragging the mouse, pressing a key on the keyboard, clicking the close-window button
on a window, or any other action that is expected to elicit a response. A listener object
has methods that specify what will happen when events of various kinds are received
by the listener. These methods that handle events are called event handlers . You the
programmer will define (or redefine) these event-handler methods. The relationship
between an event-firing object, such as a button, and its event-handling listener is
shown diagrammatically in Display 17.1 .
Event-driven programming is very different from most programming you have seen
before now. All our previous programs consisted of a list of statements executed in
order. There were loops that repeat statements and branches that choose one of a list of
statements to execute next. However, at some level, each run of a program consists of a
list of statements performed by one agent (the computer) that executes the statements
one after the other in order.
Event-driven programming is a very different game. In event-driven programming,
you create objects that can fire events, and you create listener objects to react to the
events. For the most part, your program does not determine the order in which things
happen. The events determine that order. When an event-driven program is running,

event

listener

firing an
event

event handler

2 If you have not yet covered Chapter 9 on exceptions, you can safely ignore this paragraph.